In a CDMA system, since information signals for a user are spread with a spreading code assigned for each user, the user is identified by the spreading code. That makes it possible to multiple signals of a plurality of users in the same frequency band, which results in a high efficiency in frequency utilization.
In a conventional base station in a CDMA system, it is possible to transmit information signals spread with spreading codes of different spreading factors. In such CDMA system, the number of users is the largest in the case where all users transmit information signals spread with spreading codes of the highest spreading factor, which means the information signals have the smallest symbol rate.
For instance, by making a spreading factor half, the symbol rate becomes twice, and two spreading codes of the lowest spreading factor are supposed to be consumed because of the orthogonality in codes. In other words, the total channel capacity of transmitted information does not change by combing any spreading factors.
On the other hand, since a demodulator is necessary for each user not depending on spreading factors of spreading codes, taking account into the case where all users communicate using the lowest symbol rate, the number of demodulators needed in this case are prepared. Accordingly, in the case where some users utilize spreading codes of different spreading factors, it is possible to demodulate the signals of all users, however some modulators are leftover.
FIG. 1 is a block diagram illustrating a schematic configuration of a conventional reception section at a base station in a CDMA system. And, FIG. 2 is a diagram to explain operations in the base station.
In FIG. 1, a reception section at a base station in CDMA system are primarily composed of reception antenna 1, radio section 2, and first through eighth demodulators 3, 5, 7, 9, 11, 13, 15 and 17. Data for each user are extracted from each demodulator above. For instance, user data 4 of first user are extract from first demodulator 3. In the same way, second through eighth user data 6, 8, 10, 12, 14, 16, and 18 are extracted from second through eighth demodulators 5, 7, 9, 11, 13, 15 and 17 respectively. In this case, the highest spreading factor is assumed 8.
The outline of operations of a reception section at a base station in a CDMA system is explained using FIG. 1. A reception signal received at reception antenna 1 is converted into a baseband signal at radio section 2. A user signal for a different user is extracted at either of first through eighth demodulators #1 through #8 (3, 5, 7, 9, 11, 13, 15 and 17 ) to obtain first through eighth user data #1 through #8 (4, 6, 8, 10, 12, 14, 16, and 18) respectively.
In this case, the maximum allowable user number is eight. When eight users all transmit signals using spreading codes of spreading factor 8, first through eighth demodulators #1 through #8 are all used respectively to demodulate data of first through eighth users #1 through #8. However, in the case where some users utilize spreading codes of different spreading factors, the number of used spreading codes differ and so does the information capacity. For instance, a user utilizing a spreading code of spreading factor 4 can transmit an information capacity twice than a user utilizing a spreading code of spreading factor 8. However it means the user consumes two spreading codes of spreading factor 8. Similarly, a user utilizing a spreading code of spreading factor 2 can transmit an information capacity four times than a user utilizing a spreading code of spreading factor 8. However it means the user consumes four spreading codes of spreading factor 8.
FIG. 2 is a diagram illustrating the case where users utilize spreading codes of different spreading factors. In FIG. 2, second user #2 utilizes a spreading code of spreading factor 4, which means the user utilizes two spreading codes of spreading factor 8. And, fifth user #5 utilizes a spreading code of spreading factor 2, which means the user utilizes four spreading codes of spreading factor 8. Because of it, the allowable user number is four. Accordingly, data of first user #1, data of second user #2, data of fourth user #4 and data of fifth user #5 are respectively demodulated at first demodulator #1, second demodulator #2, fourth demodulator #4 and fifth demodulator #5. However, four demodulators; third demodulator #3, sixth demodulator #6, seventh demodulator #7 and eighth demodulator # 8; are leftover(shadow parts in FIG. 2 )
In communications in a CDMA system, in the case of transmitting signals using spreading codes of different spreading factors, the communication quality of signals of a user utilizing a spreading code of low spreading factor is easy to deteriorate against delayed waves especially because the spreading gain is small. To compensate the deterioration in communication quality against delayed waves, it is necessary to increase transmission power, which brings the problems that interference against other users increases and the capacity decreases.
And, because the most appropriate demodulation method and parameter differ depending on the line conditions and others such as fading pitch and profile of delayed versions, it is difficult to assume the most appropriate demodulation method and parameter for various line conditions, for instance, what demodulation method or parameter is useful for what line condition, and so on.